The present invention relates to a thermal head having a plurality of heating elements, each being connected to a pair of lead electrodes which extend in the same direction relative to the heating element and which are connected to common electrodes.
FIG. 4 shows the conventional thermal head in which a first common electrode 23 is connected to one end of each heating element 21, and an individual lead electrode 22 is connected to the opposite end of each heating element 21. A plurality of the heating elements 21 constitute one block. Each of the individual lead electrodes 22 is connected through a corresponding switching element 25 to a second common electrode 24. Generally, the second common electrode 24 is separately provided for each block. Otherwise, a single second common electrode may be provided for a plurality of the blocks. Further, generally a multiple of the switching elements included in one block are integrated in one IC chip which is disposed on the second common electrode. The first common electrode 23 has a pair of terminals 26 disposed on opposite sides of a thermal head substrate. The second common electrode 24 has also a terminal 27 disposed on an edge area of the substrate.
However, each heating element 21 has a different resistance of the current path, dependently on its position, from the terminal 26 to the terminal 27 through the first common electrode 23, the respective lead electrode 22 and the second common electrode 24. For example, one heating element positioned centrally relative to the first common electrode 23 has a higher resistance of the current path than that of another heating element disposed farmost outside. Particularly, when concurrently driving multiple ones of the heating elements, a great amount of driving current flows through the first common electrode so that the voltage applied to the heating elements varies significantly between a central element and an end element due to the difference in their current path resistance, thereby causing variation in their thermal outputs. This thermal output variation may be increased when a great number of heating elements are driven concurrently. Therefore, the conventional thermal head would exhibit a significant variation in the dot impression density. To avoid such variation, complicated control of the electric energy is needed in the conventional thermal head, thereby increasing the production cost thereof.